1. Field of the Invention
The subject matter disclosed, generally relates to structures and methods for fabricating solid state image sensors.
2. Background Information
Photographic equipment such as digital cameras and digital camcorders may contain electronic image sensors that capture light for processing into still or video images. Electronic image sensors typically contain millions of light capturing elements such as photodiodes.
Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, photo sensors are formed in a substrate and arranged in a two-dimensional array. Image sensors typically contain millions of pixels to provide a high-resolution image.
FIG. 1A shows a sectional view of a prior art solid-state image sensor 1 showing adjacent pixels in a CMOS type sensor, reproduced from U.S. Pat. No. 7,119,319. Each pixel has a photoelectric conversion unit 2. Each conversion unit 2 is located adjacent to a transfer electrode 3 that transfers charges to a floating diffusion unit (not shown). The structure includes wires 4 embedded in an insulating layer 5. The sensor typically includes a flattening layer 6 below the color filter 8 to compensates for top surface irregularities due to the wires 4, since a flat surface is essential for conventional color filter formation by lithography. A second flattening layer 10 is provided above the color filter 8 to provide a flat surface for the formation of microlens 9. The total thickness of flattening layers 6 and 10 plus the color filter 8 is approximately 2.0 um.
Light guides 7 are integrated into the sensor to guide light onto the conversion units 2. The light guides 7 are formed of a material such as silicon nitride that has a higher index of refraction than the insulating layer 5. Each light guide 7 has an entrance that is wider than the area adjacent to the conversion units 2. The sensor 1 may also have a color filter 8 and a microlens 9.
The microlens 9 focuses light onto the photo photoelectric conversion units 2. As shown in FIG. 1B because of optical diffraction, the microlens 9 can cause diffracted light that propagates to nearby photoelectric conversion units and create optical crosstalk and light loss. The amount of cross-talk increases when there is a flattening layer above or below the color filter, positioning the microlens farther away from the light guide. Light can crosstalk into adjacent pixels by passing through either flattening layer (above or below color filter) or the color filter's sidewall. Metal shields are sometimes integrated into the pixels to block cross-talking light. In addition, alignment errors between microlens, color filter, and light guide also contribute to crosstalk. The formation, size, and shape of the microlens can be varied to reduce crosstalk. However, extra cost must be added to the precise microlens forming process, and crosstalk still cannot be eliminated.
Backward reflection from the image sensor at the substrate interface is another issue causing loss of light reception. As shown in FIG. 1A, the light guide is in direct contact with the silicon. This interface can cause undesirable backward reflection away from the sensor. Conventional anti-reflection structures for image sensors include the insertion of a oxide-plus-nitride dual-layer film stack directly above the silicon substrate, or a oxynitride layer having variation of nitrogen-to-oxygen ratio there, but only reduces reflection between the silicon substrate and a tall oxide insulator. This approach is not applicable when the interface is silicon substrate and a nitride light guide.